Temperature hysteresis, reference electrodes

The precision and accuracy of the measurement also depend strongly on the reference electrode, which affects the results through fluctuations in its own potential and through the liquid-junction potential at the test solution-liquid bridge interface. This subject is extensively treated in [158]. Common electrodes of the second kind have sufficiently stable potentials at a constant temperature, but difficulties can be encountered due to temperature hysteresis. Silver chloride electrodes are preferable to calomel electrodes, because their temperature hysteresis is substantially smaller with a calomel electrode, potential stabilization after a change in the temperature may even take several hours. Negligible temperature hysteresis is exhibited by the thallamide reference electrode [26,... 

Properties of the Ideal Reference Electrode. An ideal reference electrode should show the following properties (1) it should be reversible and obey the Nemst equation with respect to some species in the electrolyte (2) its potential should be stable with time (3) its potential should return to its initial value after small currents are passed through the electrode (no hysteresis) (4) if it is an electrode of the second kind (e.g., Ag/AgCl), the solid phase must not be appreciably soluble in the electrolyte and (5) it should show low hysteresis with temperature cycling. 

TI+/TI(Hg) electrode — A -> reference electrode commonly known as Thalamid electrode employing thallium amalgam (40wt%) as electronically conducting phase and an aqueous solution of KC1 (saturated or 3.5 M) saturated with T1C1. In comparison with the saturated calomel electrode it shows a superior temperature stability up to T = 135 °C without temperature hysteresis, no disproportionation of T1C1 (as compared to Hg2 CI2) or significant complexation are found. 

The accuracy of the silver chloride reference electrode depends upon the amount of chloride ion that is present in the solution and the accuracy with which it is controlled. In practical applications two concentrations of chloride ion are popular. In the first the electrolyte surrounding the silver chloride element is seawater. The other type of reference electrode uses a saturated sodium or potassium chloride solution. Both silver chloride cells have large temperature coefficients but there is no hysteresis or other effects so that these variations can be calculated. These reference electrodes typically have a higher resistance than the copper sulfate cells and this resistance is a function of the thickness of the silver chloride layer. 

APC) (a) and (Mg2(p-C1)3-6THF) (HMDSAICI3) (GENl) (b) on a stainless steel working electrode with an area of 0.02 cm at a temperature of 21 °C. Counterclockwise arrows designate hysteresis. Scan rate is 25 mV/s and the counter and reference electrode are both magnesium... 

The main advantage is the zero temperature coefficient of the iodine-iodide reference system and the very small hysteresis. On the other hand, this reference electrode is free of silver ions to avoid problems due to interactions with sulfide ions or proteins at the liquid junction. In harsh environments the iodine-iodide electrode is almost insensitive to small currents and to electromagnetic interferences. 

The reference electrodes that use the same solvent as the solution under study should have properties as follows (1) the potential should be reproducible and stable with time (2) it should be reversible and obey the Nemst equation with respect to the potential-determining species in the solution (3) it should return to the initial value when a small current is passed through the electrode and then stopped and (4) it should show no hysteresis with temperature cycling. The reference electrodes of this group are listed in Table 6.1, in the alphabetical order of solvents. In popular solvents such as AN, DMF, DMSO, and PC, many types of reference electrodes have been used. Some explanations are given below for each type of reference electrodes. 

The potential of the Ag/AgCl reference electrode is temperature dependent. At 25°C with 3.5 M KCl it has a value of +200 5mV. This amount is to be added to each measured EMF to refer it to the normal hydrogen electrode. If the potential of the Ag/AgCl reference electrode is determined at various temperatures between 0 and 95 C by measuring the EMF relative to a hydrogen electrode, whenever the electrode is warmed or cooled back to room temperature, the electrode potential returns to within 2mV of its original value. This temperature hysteresis effect is thus very small with the Ag/AgCl reference electrode. For this reason it is recommended for all applications in which the temperature cannot always be held constant or lies above 80°C. 

The reason that the Thalamid reference electrode is introduced here before the much more well-known Calomel reference electrode stems from its far superior temperature stability. It shows no temperature hysteresis up to 135°C, and in this respect is even superior to the Ag/AgCl reference electrode. As with the Ag/AgCl electrode, the reaction... 

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